1. Field of Invention
The invention relates to a driver circuit of an inkjet print head and, more particularly, to a driver circuit-integrated driver transistor structure of an inkjet print head and the method for making the same.
2. Related Art
The inkjet printer is a common peripheral device of a computer. There is usually a print head for ejecting ink droplets in the machine, e.g. a thermal bubble inkjet print head. The basic structure of a normal print head includes an ink channel, a nozzle and an orifice plate for ejecting ink, an actuator for ink ejection and a proper driver circuit. When the inkjet printer is printing, the ink is propelled by the actuator, such as a heater, and is ejected from the nozzle on the orifice plate to form ink dots on paper. Generally speaking, the thermal bubble inkjet print head uses a heater as the actuator device, which heats up the ink in the ink channel to produce thermal bubbles to jet the ink.
In order to improve performance in terms of resolution and printing speed, one needs a large number of nozzles on each inkjet print head. Currently, the thermal bubble inkjet print head uses a design with serial driver transistors and heaters. An active driver array is incorporated in the driver circuit and is integrated into the circuit structure of the inkjet print head chip. This is the so-called IDH (integrated driver head) chip. If there are N electrical joints between the inkjet print head chip and the printer, the chip can drive and control (N/2)2 nozzles. The above mentioned driver transistor is a current driver. It has to adopt a comb or grating MOSFET gate structure, or a bipolar transistor base structure to connect several sets of transistors in parallel. As shown in FIG. 1, the driver transistor structure has several MOSFET elements 21 connected in parallel. Each MOSFET element includes a source region 211, a drain region 212 and a gate 213. The gates 213 of the MOSFET elements are connected in parallel to form a comb gate structure 22. A body contact region 20xe2x80x2 is formed outside the active region 20. The body contact region 20xe2x80x2 is formed with a plurality of body contacts (or substrate contacts) 23. The locations and areas of the body contacts 23 can be defined by the barrier layer 24 of a polysilicon doped layer. In the prior art, the body contacts 23 and the source of the MOSFET element maintain electrical contact to maintain the substrate of the MOSFET element at the lowest level or ground. The driver transistor structure uses tetraethosiloxane (Si(OC2H5)4, TEOS) silicon oxide, PSG, or BPSG (Boron Phosphorus Silicon Glass) as an interlayer dielectric by CVD (Chemical Vapor Deposition). The interlayer dielectric is etched to form contact holes 26 of gates, drains, sources and body contacts.
To supply a sufficient driving current, the driver transistor structure adopts the MOSFET design of a large channel W/L (Width-to-Length) ratio. The width of the active region 20 has to be between 400 micrometers and 900 micrometers to provide a working voltage of 10V and a working current above 200 mA. However, such a design makes the active region far from the body contacts (over 400 micrometers). This cannot guarantee that all channels in the MOSFET elements inside the active region are perfectly grounded, resulting in secondary breakdowns and lowering the tolerance of the elements. As to the manufacturing and structure of the driver transistor of a conventional 300 dpi or 600 dpi IDH chip, the heater, MOSFET elements, and field region with body contacts are integrated together. The body contacts are installed in the thick oxide field layer (with a thickness between 9000 A to 17500 A). In this structure, a basic body contact structure is about 15xc3x9715 xcexcm2, excluding the gaps in between. A MOS driver transistor structure is roughly 80xc3x97600 xcexcm2, excluding the body region. 18 body contacts along with the gaps in between occupy 80xc3x97150 xcexcm2. On the average, each driver transistor provides ⅙ to ⅓ of its area for the body contact region of the field oxide. The body contact occupies a large portion of the area.
Current products usually have 200 to 400 driver transistors on an inkjet print head. These driver transistors occupy a large portion of the area in the chip. With the increase of resolution of the inkjet print head, the number of driver transistors on a single inkjet print head chip has to be increased along with the number of heaters and nozzles. Although scaling down the MOSFET elements can accommodate more driver transistors in a unit area, the scaled-down MOSFET elements and other loops have higher parasitic resistance and the heat generated from each unit area also increases. Therefore, it requires a higher chip manufacturing cost.
Thus, how to minimize the area occupied by each driver transistor without decreasing the sizes of MOSFET elements while increasing the reliability of elements in the driver transistor structure design of an inkjet print head chip is a subject worth further research and exploration.
In view of the foregoing, an objective of the invention is to provide a driver transistor structure of an inkjet print head chip and its manufacturing method. The invention can lower the resistance RB from the MOSFET channel in the active region to the body contact, avoiding secondary breakdowns and increasing element reliability.
Another objective of the invention is to provide a driver transistor structure of an inkjet print head chip and its manufacturing method that can minimize the area occupied by each driver transistor on the inkjet print head chip without increasing parasitic resistance and manufacturing costs.
To achieve the above objectives, the invention distributes several body contacts in a large area MOSFET active region so that the equivalent resistance RB between the MOSFET channel and the body-contact greatly decreases as the distance is reduced. Therefore, it can prevent the occurrence of secondary breakdowns. Furthermore, the body contacts are installed in the active region of the driver transistor structure. For example, the body contacts are embedded in the source, the so-called BES (Body-contact Embedded in Source) structure, without defining in advance the body region and making the body contacts in the field oxide region outside the active region. Accordingly, such a BES MOSFET driver transistor structure can save about 20% area without decreasing the sizes of MOSFET elements in the active region. This method can also increase the number of inkjet print head chips on each wafer, thus lowering the average manufacturing cost of each chip.
In accordance with the disclosed driver transistor structure of an inkjet print head chip, at least one body contact is installed in an active region of the driver transistor. The active region has a plurality of MOSFET elements connected in parallel. These MOSFET""s are used to control an ink actuator (e.g. current supply of a heater) in electrical contact with the driver transistor in the inkjet print head chip. The body contact can be embedded in or next to the source of the MOSFET element. The minimum distance between the dopant region of the body contact and the region of the source region with another type of dopant can be less than 5 xcexcm. The body contact and the source of the MOSFET element in the active region are connected using a conductor to keep them at the same level.
According to the disclosed manufacturing method of the driver transistor of an inkjet print head chip, at least one body contact is installed in the active region of the driver transistor. The method forms at least one dopant barrier layer to define a dopant barrier region during the formation of the MOSFET element in the active region. The dopant barrier layer is used to prevent drain and source dopants (e.g. N+ dopants) from entering the dopant barrier region during the diffusion or ion implantation process. Afterwards, the dopant barrier layer is etched to define a dopant region for body contact. In the dopant region of body contacts, a body-contact dopant of a type opposite to the drain and source dopant is implanted in the body contact dopant region by ion implantation or diffusion to obtain the body-contact.
In particular, the dopant barrier layer can be a polysilicon layer or other materials that can stop or resist dopants, for example, a dielectric layer, refractory metal or alloy will work. The dopant barrier layer can be formed while depositing the gate polysilicon in the MOSFET element or during another deposition or coating process. Furthermore, the region of the dopant barrier layer can be defined by an etching step the same as or different from the gate polysilicon layer.